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July 11, 2011 -- Babies born to moms who smoke are more likely to have certain birth defects compared to infants with mothers who don't smoke during pregnancy, a large new study shows.

The study, a fresh look at 50 years of research, is the first scientific review of studies that have looked at the connection between smoking and birth defects. It includes information on nearly 12 million infants, including 173,000 that were born with malformed bodies.

"This is a seminal paper," says Michael Katz, MD, senior vice president of research and global programs for the March of Dimes and a professor emeritus at Columbia University.

Katz says previous studies have revealed piecemeal links between maternal smoking and birth defects, particularly the risk for cleft lips and cleft palates. But none has ever revealed such a comprehensive picture of the harms smoking is likely to cause.

"This is an encyclopedic review," Katz tells WebMD.

Smoking and Birth Defects

Experts have long suspected that smoking cigarettes was unhealthy for mother and baby. Indeed, cigarette labels already include the federally mandated warning that "smoking during pregnancy can harm your baby."

The new study shows how smoking is associated with increases in specific risks.

For the review, researchers culled 50 years of research on smoking and birth defects, narrowing their focus to just 172 studies.

They extracted data from those studies and pooled the numbers to create a big-picture look at birth defects related to cigarette smoking.

Babies born to mothers who smoked had roughly 20% to 30% higher odds of having shortened or missing arms and legs, cleft lips and cleft palates, and abnormally shaped heads or faces compared to babies born to nonsmoking mothers.

"These defects last a lifetime," says study researcher Allan Hackshaw, deputy director of the University College London Cancer Center in the U.K. "They can be fixed to some extent, but they're visible on the baby and infants and more or less for life."

Maternal smoking was associated with a 27% excess risk of gastrointestinal abnormalities, including problems with the throat, esophagus, colon, intestine, bile ducts, gall bladder, and liver.

Additionally, infants born to smokers had 50% higher odds of being born with their intestines hanging outside the body and a 20% increased risk of being born with a blocked or closed anus.

Smoking accounted for a 9% increased risk of heart defects and a 13% higher risk that baby boys would be born with undescended testes.

How Cigarettes May Harm Developing Babies

Part of the problem may be that the baby isn't getting enough oxygen when mom smokes.

Nicotine, the addictive substance in cigarettes, causes blood vessels to constrict, which may choke off the supply of oxygen to the fetus.

And carbon monoxide, a colorless, odorless gas that's present in high levels in cigarette smoke, binds more strongly to hemoglobin -- the molecule that ferries oxygen through the blood -- than oxygen itself, lowering the amount available to the baby.

"It personally absolute stuns me that some people still smoke during pregnancy," Hackshaw says.

Studies show 14% of women in the U.S. smoke while pregnant. That number jumps to 20% expectant mothers under age 25.

"The advice has to be 'don't smoke'," says Katz.

Though smoking is a tough habit to kick, he says many women do successfully quit after they find out they're pregnant.

"The motivation is much stronger in pregnant women than in the population at large," he says. "It's less difficult to persuade people who are pregnant to stop smoking than anyone else."